1. Field of Invention
The present invention relates generally to power conversion and, more generally, to DC-DC switch-mode power converters.
2. Description of the Background
DC-to-DC power converters are power-processing circuits that convert an unregulated input DC voltage to a regulated DC output voltage. There are two basic methodologies for accomplishing regulated power conversion. The first is called xe2x80x9clinear regulationxe2x80x9d because the regulation characteristic is achieved with one or more semiconductor devices operating in the linear region. Linear regulators provide the advantages of simplicity, low output noise, fast response times, and excellent regulation. They may, however, be unacceptably inefficient in certain applications.
The second methodology is called xe2x80x9cswitch-modexe2x80x9d conversion, which, in contrast to linear regulation, offers the powerful advantage of high efficiency. In this case, the voltage conversion is achieved by switching one or more semiconductor devices rapidly between their xe2x80x9conxe2x80x9d (or conducting) state and their xe2x80x9coffxe2x80x9d (or non-conducting) state such that the appropriate amount of energy is transferred to the load. This principle is called pulse width modulation (PWM).
Switch-mode DC-to-DC power converters typically include an inverter, a transformer having a primary winding coupled to the inverter, and a rectifier circuit coupled to a secondary winding of the transformer. The inverter typically includes an actively controlled semiconductor switch that converts the DC input voltage to an alternating voltage, which is magnetically coupled from the primary winding of the transformer to the secondary winding. The rectifier circuit rectifies the alternating voltage on the secondary winding to generate a desired DC output voltage. An output filter is also typically included to smooth the output voltage and/or current.
To achieve high efficiency and high performance power conversion, it is desirable to use low voltage rating switching devices for better switching and conduction characteristics. It is also desirable to employ converter circuits with relatively continuous power transfer to alleviate the need for heavy filtering for the output and/or input.
One known switch-mode converter is the single-ended forward converter with a passive reset circuit. Such a circuit topology, a resonant-reset forward converter, is illustrated in FIG. 1. When the primary side power switch Q1 is turned on, the input voltage Vin is coupled to the secondary side of the converter through the transformer T1. The secondary side voltage is rectified to provide the DC output voltage Vout. When the primary side power switch Q1 is turned off, the magnetizing flux of the transformer T1 is reset by the voltage appearing on the resonant capacitor Cr, and the output choke current free wheels through the rectifier D2. Typical waveforms for the input current Iin and the rectified voltage Vrec for the resonant-reset forward converter of FIG. 1 are illustrated in FIG. 2.
The major drawback of this type of converter is that the voltage stress on the semiconductor devices, such as the switch Q1, is very high. Thus, semiconductor switches with higher voltage ratings ordinarily have to be utilized. In addition, the resonant-reset forward converter is not very efficient with synchronous rectification, discussed later, especially for wide input voltage ranges and large load variations.
Another known switch-mode converter, the active-clamp forward converter, is illustrated in FIG. 3. This type of converter includes a series-connected reset switch Q2 and a resonant capacitor Cr connected in parallel with a winding of the transformer T1, in this case the primary winding. The reset switch Q2 and capacitor Cr form a xe2x80x9creset circuitxe2x80x9d that actively resets the transformer T1. Typical waveforms for the input current Iin and the rectified voltage Vrec for the active-clamp forward converter of FIG. 3 are illustrated in FIG. 4.
The active-clamp forward converter reduces the voltage stress on the active switching elements (such as the switches Q1 and Q2), thereby permitting the usage of low voltage rating devices. However, as far as the input current Iin and output voltage Vout are concerned, both the resonant-reset forward converter and the active-clamp forward converter have pulsating input and output power as illustrated in FIGS. 2 and 4, respectively, which necessitate bulky filtering components.
Another known switch-mode converter, the forward-flyback converter, allows the transformer flux to operate under a dc bias condition and has a continuous rectified output voltage. Its input current, however, remains pulsating. It is also known to use a separate boost inductor with a half-bridge converter to achieve both smooth input current and output voltage. The boost inductor, however, is generally bulky in order to achieve the continuous-current mode operation.
Another aspect to achieve high efficiency for switch-mode converters has been the replacement of the conventional rectifier diodes in the rectifier circuit (such as the diodes D1 and D2 in the converters of FIGS. 1 and 3) with MOSFETs, which have extremely low conduction losses. The xe2x80x9cself-drivenxe2x80x9d scheme of synchronous rectification, which uses the secondary winding voltage to drive the rectifier MOSFET directly or feed a gate driver circuit for the MOSFET, is known to be simple, effective, and cheap. In order to use such a self-driven mechanism, it is preferable that the winding voltage on the secondary side be well balanced in the whole operating range.
In view of the preceding, there exists a need in the art for a high efficiency and cost-effective switch-mode converter that uses low voltage stress semiconductor devices, provides smooth power transfer without bulky filters, and is able to use the effective self-drive technique for the rectifier MOSFETs.
The present invention is directed to a DC-DC switch-mode power converter. According to one embodiment, the converter includes a transformer having first and second series-connected primary windings, a first capacitor connected in series to the second primary winding, a first switch for cyclically coupling an input voltage to the first and second primary windings, a second capacitor, and a second switch for cyclically coupling the first and second primary windings to the second capacitor. The converter may also include a rectifier circuit coupled to the transformer, wherein the rectifier circuit includes a pair of self-driven synchronous rectifiers.
According to another embodiment, the converter includes a boost converter, including the first primary winding of the transformer, the first and second switches which, when energized alternately, create a current in the first primary winding, and the second capacitor connected to the second switch. In addition to the boost converter, the converter also includes an asymmetrical half-bridge converter, including the second primary winding, the first and second capacitors, the first and second switches, a secondary winding of the transformer, and a secondary circuit coupled to the secondary winding. The secondary circuit may include a rectifier circuit including a pair of self-driven synchronous rectifiers.
Embodiments of the present invention provide many advantages and improved features relative to prior art switch-mode power converters. For instance, the present invention allows low voltage rating MOSFETs with improved switching and conduction characteristics to be utilized, thus providing enhanced efficiency. An additional feature of the present invention is that the output voltage is continuous. As a result, smaller output filter components may be utilized. A further feature of the present invention is that the input current has less ripple components without using an extra magnetic component; therefore, small input filter components may be utilized. Additionally, self-driven synchronous rectifiers may be used for the rectifier circuit of the present invention, thereby realizing the enhanced efficiency associated therewith. Furthermore, the second primary side switch may turn on at a zero voltage condition, and the first primary side switch, which processes the full input power, turns on at lower voltage stress, thus realizing further efficiency benefits. These and other benefits of the present invention will be apparent from the detailed description to follow.